This invention relates generally to compact print wheel printers and more particularly to a compact print wheel printer having low operational noise and low power consumption with a superior, life extended character selection capability.
In small, compact print wheel printers, such as the type employed in calculators, cash registers and the like, there is formed a single indent in the outer surface of the print wheel shaft of the printer. The angle of the forward surface or forward edge of this single print wheel shaft indent relative to the direction of rotation, A, of the print wheel shaft is provided to permit an extension of a print wheel spring held about the print wheel shaft to slip into and engage the shaft indent so that the configuration of indent is fixed regardless of the depth of the indent and the angle of the print wheel spring extension which slips into and engages the indent is also fixed.
Further, the print wheel shaft indent is provided only for positioning and maintaining a releasably fixed relationship of movement or unitary motion between the print wheel shaft and the print wheel until the print wheel is stopped at a selected position by means of a character selection pawl wherein the desired character or font on the periphery of the print wheel is printed. When the print wheel is stopped by the pawl, the spring extension is released from the shaft indent so that the print wheel no longer rotates in unison with the print wheel shaft.
FIGS. 4A and 4B disclose the prior art arrangement of a print wheel 3, print wheel shaft 1 and print wheel spring 2 of a compact print wheel printer of the foregoing described type. Print wheel shaft 1 has an indent 1' on its surface wherein the extension 2' of spring 2 may engage and bring about a fixed engagement between shaft 1 and print wheel 3 since spring 2 is fixed at its other end to print wheel 3.
As shown in FIG. 4C, surface 1" of indent 1', onto which print wheel spring 2 slips and engages, is parallel to the direction, E, of the motion of print wheel spring 2 as held about print wheel shaft 1 and the angle of print wheel spring extension 2' is also parallel to this same surface 1". The direction of motion, E, of spring extension 2' is substantially radial relative to the center of shaft 1. As a result, when the print wheel spring extension 2' slips into shaft indent 1', a a high force impact is created which is accompanied with a high noise level due to the spring extension 2' falling the full depth of shaft indent 1' in one continuous and uninterrupted movement, which causes, over time, excessive wear on print wheel spring 2. This has been remedied, in part, by manufacturing spring 1' with higher strength material. However, this, in turn, causes excessive wear on shaft indent 1' and results in an even greater noise level of operation.
As shown in FIG. 4B, and more particularly in enlarged detail view of FIG. 4C, surface 1", upon which print wheel spring 2 first engages and falls onto shaft indent 1', becomes excessively worn, as shown at 2" and, therefore, is no longer parallel to the above mentioned direction, E. As a result, spring extension 2' is no longer properly and accurately positioned in shaft indent 1'. Further, there results a degradation in the character selection function since the engagement of the end of spring extension 2' along this surface 1" is no longer smooth and is interrupted by the formed irregular worn surface 2". Thus, accurate positioning and degradation of the character selection function occurs because for accurate positioning, it is necessary for print wheel spring extension 2' to slide along a smooth surface 1" to reach the full depth of shaft indent 1' since ultimate fine print wheel positioning for proper character printing is performed, in part, by the cooperative relationship of the bottom radius of indent 1' and the end radius of print wheel spring extension 2'.
Furthermore, only one such indent 1' is provided in print wheel shaft 1' for maintaining the unitary motion of print wheel shaft 1 and print wheel 3 during the character selection process. As a result, print wheel spring 2 has to be constructed of high strength and durable material to permit the print wheel printer to accommodate high speed printing. This results in increased noise when the print wheel spring 2 slips into and engages in print wheel shaft indent 1' and, further, requires a greater drive force to be placed upon print wheel shaft 1, which, in turn, increases the resultant power consumption necessary for high speed printing operation of the printer.
It is a principal object of this invention to solve the foregoing mentioned disadvantages of print wheel printers of the prior art.
It is another object of this invention to provide means by which the foregoing mentioned excessive wear can be significantly reduced while also significantly reducing the level of noise produced by the print wheel printer.
It is still another object of this invention to provide a compact print wheel printer having low operational noise and low power consumption with a sustained or life extended, superior and accurate character selection capability.